The present description relates to a spark ignited internal combustion engine, and more particularly to one having a relatively high geometric compression ratio.
When a geometric compression ratio of an internal compression ratio is increased, its expansion ratio is increased as well. The greater expansion ratio can convert heat which the combustion of air fuel mixture has generated more effectively into a downward movement of the piston. Therefore, engines with greater compression ratios operate more efficiently. On the other hand, the greater geometric compression ratio may excessively increase a temperature in the cylinder around a compression top dead center. Under the high temperatures in the cylinder, fuel residing away from the spark plug may ignite by itself before it is reached by the flame. Under the excessively high temperatures, the fuel in the cylinder may ignite by itself even before the spark ignition. Such phenomena are commonly known as knocking and pre-ignition.
One way to avoid such abnormal combustion as knocking and pre-ignition is reducing volume of air charged in a cylinder. It is known, and for example described in U.S. Pat. No. 6,626,164, an intake valve is controlled to close later after bottom dead center of the intake stroke during a cylinder cycle to reduce the air volume charged in the cylinder in order to avoid abnormal combustion.
The prior method can reduce the cylinder air charge without reducing engine operating efficiency. Specifically, it does not need to throttle the air flowing through the intake passage to reduce the cylinder air charge. Therefore, a pressure in the intake passage, and eventually a pressure in the cylinder during an intake stroke can be maintained. As a result, a pressure working on the upper side of the piston when it descends is greater, which means a lesser pumping loss and a greater engine operating efficiency.
Such abnormal combustion caused by a higher temperature in the cylinder is more likely to occur when the engine speed is lower, the engine temperature is higher, the intake air temperature is higher, or the intake air humidity is lower. Therefore, it is preferable, in such operating conditions, to close the intake valve later after the bottom dead center during a cylinder cycle. On the other hand, in engine operating conditions such as when the engine speed is higher, the engine temperature is lower, or the intake temperature is lower, such abnormal combustion is less likely to occur. Therefore, under these operating conditions, it is preferable to charge more air into a cylinder to increase output torque from the engine, and to close the intake valve around bottom dead center. As a result, in order to secure sufficient power output from an engine with a greater compression ratio while avoiding abnormal combustion, it is necessary to broadly vary the closing timing of the intake valve.
It is necessary to broaden the varying range of the intake valve closing timing if the geometric compression ratio is very high for the reasons described above. However, it has not been known how much range is enough, and it has been usually thought that an excessively broad range is necessary. Such an excessively broad varying range of the intake valve closing timing is not acceptable because the closing timing may not vary quick enough to respond to a change of the cylinder air charge demand.
Currently, having considered the above setbacks, spark ignited internal combustion engines commercially available are not designed to have greater compression ratios and greater operating efficiency derived from the greater expansion ratios.
Therefore, there is room to improve an operating efficiency of a spark ignited internal combustion engine without causing abnormal combustion.